A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).
As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore's law’, To keep up with Moore's law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.
In order to control the lithographic process to place device features accurately on the substrate, marks may be provided on the substrate, and the lithographic apparatus may include one or more alignment sensors by which positions of such marks on a substrate can be measured accurately. These alignment sensors are effectively position measuring apparatuses. Different types of alignment marks and different types of alignment sensors are known e.g. are provided by different manufacturers. In general, an alignment sensor has a radiation source which provides a beam of radiation of one or more wavelengths which is projected onto a mark located on a substrate. Radiation diffracted by the mark is collected and a position of the mark is determined from this diffracted radiation.
It is an object of the present invention to provide an alternative radiation source suitable for use in an alignment sensor which at least partially addresses one or more problems associated with prior art radiation sources, whether identified here or not.